Pulmonary thromboembolism (PE) with circulatory and/or respiratory symptoms is associated with high morbidity and mortality. Acute pulmonary hypertension is the hallmark of severe PE, and is to be held responsible for the full spectrum of clinical manifestations and complications. Although it is common belief that only mechanical obstruction by thrombus mass causes this pulmonary hypertension, there is strong evidence indicating that pulmonary vasoconstriction contributes significantly to the rise in pulmonary vascular resistance.